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This study provides the first extensive data on in situ rates of N2 fixation in minimally disturbed Posidonia oceanica meadows and their temporal and spatial variability along the Mallorcan coasts in the Mediterranean Sea. Benthic bell‐jar chambers were deployed underwater enclosing (1) natural P. oceanica shoots, (2) matted sediment of P. oceanica with the leaves carefully removed by clipping to assess the contribution of N2 fixation associated with leaves, and (3) adjacent unvegetated sediment to assess the contribution of N2 fixation by the sediment. In all of the chambers, N2 fixation varied seasonally, with highest rates during summer and lowest in winter. Average N2 fixation rates in chambers enclosing natural P. oceanica shoots ranged from 0.06 mg N m−2 d−1 to 1.51 mg N m−2 d−1 (0.004–0.107 mmol N m−2 d−1) in summer. Careful removal of leaves generally reduced N2 fixation rates by 16–89%. The N2 fixation rates in unvegetated sediments ranging from 0.01 mg N m−2 d−1 to 1.99 to mg N m−2 d−1 (7.1 × 10−4–0.14 mmol N m−2 d−1) have similar magnitudes in sediments with P. oceanica. This suggests that these sediments are “hot spots” for N2 fixation. Exploratory analyses of environmental factors that may influence N2 fixation rates highlight temperature and pore water total phosphorus concentrations as important factors.

The phytoplankton community composition, structure, and biomass were investigated under stratified and oligotrophic conditions during summer for three consecutive years in the Mediterranean Sea. Our results reveal that the phytoplankton community structure was strongly influenced by vertical stratification. The thermocline separated two different phytoplankton communities in the two layers of the euphotic zone, characterized by different nutrient and light availability. Picoplankton dominated in terms of abundance and biomass at all the stations sampled and throughout the photic zone. However, the structure of the picoplanktonic community changed with depth, with Synechococcus and heterotrophic prokaryotes dominating in surface waters down to the base of the thermocline, and Prochlorococcus and picoeukaryotes contributing relatively more to the community in the deep chlorophyll maximum (DCM). Light and nutrient availability also influenced the communities at the DCM layer. Prochlorococcus prevailed in deeper DCM waters characterized by lower light intensities and higher picophytoplankton abundance was related to lower nutrient concentrations at the DCM. Picoeukaryotes were the major phytoplankton contributors to carbon biomass at surface (up to 80%) and at DCM (more than 40%). Besides, contrarily to the other phytoplankton groups, picoeukaryotes cell size progressively decreased with depth. Our research shows that stratification is a major factor determining the phytoplankton community structure; and underlines the role that picoeukaryotes might play in the carbon flux through the marine food web, with implications for the community metabolism and carbon fate in the ecosystem.

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This study seeks to elucidate the spatial and temporal patterns of toxic phytoplankton in coastal waters of the Balearic Archipelago (NW Mediterranean Sea) in a multivariate approach. We also examine whether there is a direct relationship between the abundance of toxic phytoplankton and the physical-chemical parameters and different human pressure activities. Its magnitude and the species-specific pattern are also evaluated. The study is based on chlorophyll a and phytoplankton composition and abundance, as well as physical and chemical data, to evaluate the ecological status of Balearic waters for the implementation of the Water Framework Directive. A total of 24 taxa included in the Intergovernmental Oceanographic Commission toxic algae checklist and five well-known bloom formers were identified. Principal Component Analysis (PCA) for physical-chemical and human uses revealed the larger phytoplankton variability determined by seasonal patterns. Higher nutrient winter samples were distinguished from those of high transparency and lower nutrient summer waters. The Redundancy Analysis (RDA) separated the toxic species into two groups: Pseudo-nitzschia species, related to winter; and dinoflagellates and other flagellates species, related to spring and summer. The among-islands effect revealed a latitudinal gradient, especially regarding nutrient and human pressures related to Mallorca Island. Meanwhile, the Formentera waters were transparent and far from human activities.